drm/i915: Introduce g4x_dp.c

Move the g4x+ DP code into a new file. This will leave mostly
platform agnostic code in intel_dp.c. Well, the misplaced phy
test stuff pretty much ruins that, but let's squint real hard
for now.

v2: Add comment exlaining which platforms are covered (Daniel)
    Leave intel_dp_unused_lane_mask() be since it is pretty generic

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318161015.22070-6-ville.syrjala@linux.intel.com